Copolymers of tert-butylstyrene, a process for making the copolymers, and use of the copolymers as additives to liquid hydrocarbon fuels

Description

[0001] This invention relates to novel copolymers derived predominantly from tert-butylstyrene, to a process for making the copolymers and to the use of the copolymers as additives to liquid hydrocarbon fuels, typically aviation kerosene, for the purpose of reducing the tendency of such fuels to form inflammable mists when subjected to shock.

[0002] According to a first aspect of the invention, there is provided a copolymer of (i) at least 75% by weight of tert-butyl-styrene, (ii) from 1 % to 24% by weight of a second monomer selected from the acrylic and methacrylic esters of aliphatic monohydric alcohols containing from 1 to 4 carbon atoms, 2- ethoxyethyl methacrylate, acrylonitrile, vinyl acetate, styrene and vinyl toluene, and (iii) from 1% to 10% by weight of methacrylic acid, the aggregate of monomers (i), (ii) and (iii) being 100%, the copolymer having the following characteristics:

(a) it is soluble in AVTUR 50 aviation kerosene

(b) a 0.3% by weight solution of the copolymer in AVTUR 50 aviation kerosene has a relative viscosity in the range 1.3 to 2.1 and a differential orifice flow rate in the range 3 to 6 cm³ per 30 seconds,

the terms "soluble", "AVTUR 50", "relative viscosity" and "differential orifice flow rate" having the respective meanings which are hereinafter defined.

[0003] By "soluble in AVTUR 50" we mean that solutions of the copolymer in AVTUR 50, at all concentrations in the range 0.05% to 1% by weight, are, notwithstanding that they may appear hazy or opalescent, nevertheless homogeneous in the sense that no separation from them of a swollen polymer phase occurs on standing at 20°C.

[0004] By "AVTUR 50" we mean a liquid hydrocarbon fuel complying with U.K. Government Specification D. Eng. RD 2494 (NATO Code No. F-35). AVTUR 50 has a flash point not lower than 37.8°C (100°F), and normally has a viscosity of 1.0-1.5 cp (0.10-0.15 Pa.s) at 25°C.

[0005] By "relative viscosity" we mean the ratio of (i) the viscosity of the 0.3% by weight copolymer solution in AVTUR 50 when measured by the method of British Standard No. 188:1937 "The Determination of the Viscosity of Liquids in C.G.S. Units", Part 2, using a U-tube viscometer, Size A, at 25°C, to (ii) the viscosity of AVTUR 50 when measured under those same conditions.

[0006] By "differential orifice flow rate" we mean the difference between (a) the flow rate of a 0.3% by weight solution of the copolymer in AVTUR 50 through a passage of circular cross-section having a square-edged orifice, the passage having a length of 1.574 mm (0.062 inches) and a diameter of 0.635 mm (0.025 inches), and (b) the flow rate through the same said passage of a Newtonian liquid having the same viscosity as that of the copolymer solution referred to in (a) when the said viscosities are measured by the method of British Standard No. 188:1937, the flow rates being expressed as the volume of liquid in cm³ which passes through the orifice during the second period of 30 seconds of flow. Apparatus suitable for carrying out the differential orifice flow rate determination may be constructed by appropriately modifying a type A cup according to British Standard No. 1733.

[0007] As already stated, the copolymers of the invention contain (i) a minimum of 75% by weight of units derived from tert-butyl styrene and (iii) from 1% to 10% by weight of units derived from methacrylic acid. The monomer constituent (ii), as defined above, which makes up the balance of the units of the copolymer and is present in an amount of from 1% to 24% by weight of the total, may be a single monomer selected from the list hereinabove given, or a mixture of two or more such monomers provided that the combined amounts thereof lie within the aforesaid range. The preferred monomer constituent (ii) is methyl methacrylate.

[0008] Particularly useful copolymers according to the invention as hereinbefore defined are those of (i) from 75% to 90% by weight of tert-butyl-styrene, (ii) from 7% to 15% by weight of methyl methacrylate and (iii) from 3% to 10% of methacrylic acid. Preferred copolymers within this class are those of (i) from 81% to 85% of tert-butyl-styrene, (ii) from 9% to 11% of methyl methacrylate and (iii) from 6% to 8% of methacrylic acid.

[0009] For reasons which will become apparent later, especially useful copolymers according to the invention are those as hereinabove defined which have a glass-rubber transition temperature (Tg) above 40°C.

[0010] Preferably, copolymers according to the invention as hereinabove defined have a differential orifice flow rate in the range 4.0 to 5.5 cm³ per 30 seconds.

[0011] The copolymers of the invention are most conveniently made by the aqueous emulsion polymerisation of the constituent monomers in the presence of a free radical initiator, especially a 'redox' initiator system such as a combination of ammonium persulphate and sodium dithionite.

[0012] Thus according to a second aspect of the invention there is provided a process for the production of a copolymer which comprises the emulsion polymerisation in an aqueous diluent of a mixture of monomers consisting of (i) at least 75% by weight of tert-butyl-styrene, (ii) from 1% to 24% by weight of a second monomer selected from the acrylic and methacrylic esters of aliphatic monohydric alcohols containing from 1 to 4 carbon atoms, 2-ethoxyethyl methacrylate, acrylonitrile, vinyl acetate, styrene and vinyl-toluene, and (iii) from 1% to 10% by weight of methacrylic acid, the aggregate of (i), (ii) and (iii) being 100%, the copolymer having the characteristics of solubility, relative viscosity and differential orifice flow rate in AVTUR 50 hereinbefore defined and the polymerisation being characterised by the following features:-

(a) the aqueous diluent is a mixture of acetone and water in the ratio of from 1:9 to 1:2 by weight;

(b) the ratio of the weight of monomer mixture being polymerised to the weight of aqueous diluent is from 1:9 to 1:1.5;

(c) there is present in the reaction mixture an anionic surface-active agent at a concentration in the range 1% to 10% of the weight of monomer mixture being polymerised;

(d) the reaction mixture is stirred at a temperature between 20°C and 45°C for a period of from 6 to 10 hours in the presence of a nitrogen atmosphere;

(e) there is added to the reaction mixture at some point during the first hour, in an amount of 0.05% to 0.3% based on the weight of monomer mixture, a redox initiator;

(f) there is added to the reaction mixture when polymerisation of the monomers is. complete, in an amount of 0.001 to 0.1 % based on the weight of monomer mixture taken, a chain transfer agent.

[0013] With respect to the foregoing characterising features of the process, the following preferences apply:

(a) the aqueous diluent is a mixture of acetone and water in the ratio of 1:4 by weight;

(b) the ratio of the weight of monomer mixture to the weight of aqueous diluent is 1:4;

(c) the anionic surface active agent is sodium dioctyl sulphosuccinate used at a concentration of 2.5% of the weight of monomer mixture;

(d) the reaction mixture is stirred at a temperature in the range 25-30°C;

(e) the redox initiator is a mixture of ammonium persulphate and sodium dithionite, in the amounts of 0.05% and 0.075% respectively based on the weight of monomer mixture, and is added during the first 5 to 15 minutes of reaction time;

(f) the chain transfer agent is n-octyl mercaptan, in an amount of 0.005% based on the weight of monomer mixture taken.

[0014] The above preferred features may be observed either individually or together in groups of two or more, in carrying out the process of the invention.

[0015] In defining the process of the invention in the above terms, we assume that the person skilled in the art will follow the general procedure which is customary in carrying out aqueous emulsion polymerisations, and will also observe the usual precautions particularly in regard to the exclusion of contaminants such as transition metal compounds and reducing agents which may substantially influence the initiation mechanism and hence the course of the polymerisation.

[0016] The copolymer particles obtained by means of the process described above may be isolated from the emulsion in ways which are well known in the art. A particularly suitable method of isolation is that of spray-drying, for those (the great majority) of the copolymers as hereinbefore defined which have glass-rubber transition temperatures above 40°C.

[0017] Copolymers according to the invention are of especial interest as additives to liquid hydrocarbon fuels, in particular to aviation fuels, whereby the tendency of such liquids to disseminate when subjected to conditions of shock may be controlled.

[0018] It is known that when a liquid with a free surface is subjected to conditions of shock there is a tendency for the liquid to become disseminated in particulate form and that the effect of shock may be such as to convert a proportion of the liquid into a dispersion of fine liquid droplets in air, i.e. a mist.

[0019] It is very desirable to be able to control the extent to which such a dispersion or mist of liquid is formed under shock conditions since, for example, this mist, if inflammable, may constitute a hazard. A situation in which it is most important to keep to a minimum the formation of such mist under shock conditions is the crash of an aircraft carrying inflammable liquids, such as its fuel. Though hydrocarbon fuels now used for aircraft gas turbine engines may be of a higher flash point than aviation gasoline as used in spark-ignition engines, with a consequent reduction in the risk of fire due to ignition of vapour, nevertheless mists of fuels with flash points of 32.8°C (90°F) and higher are highly susceptible to ignition by flames, electrical sparking or the effect of friction, as well as by the presence of hot metal in the engines. There is therefore a considerable fire hazard immediately after a crash of an aircraft using such fuel. Furthermore, there is the risk of propagation of fire to the bulk of liquid even if little damage is caused by ignition of the mist itself.

[0020] We have found that the tendency to particulate dissemination under shock conditions of a liquid hydrocarbon fuel suitable for use in gas turbined aircraft and having a flash point of at least 32.8°C (90°F) may be reduced by dissolving in the liquid a copolymer of the kind described above, in a concentration of from 0.05% to 1.0% by weight.

[0021] Thus according to a third aspect of the invention we provide a modified liquid hydrocarbon fuel of flash point at least 32.8°C (90°F) suitable for use in gas turbine engined aircraft, and having a reduced tendency to particulate dissemination on being subjected to shock, the fuel containing dissolved therein from 0.05% to 196 by weight of a copolymer of tert-butyl-styrene as hereinabove defined.

[0022] Preferably the fuel contains from 0.2% to 0.5% by weight of the dissolved copolymer.

[0023] A liquid hydrocarbon fuel which is of particular interest for modification according to the invention is AVTUR 50 aviation kerosene as hereinabove defined, but other suitable fuels include aviation turbine fuels JP-8 (flash point 43.3°C (110°F) min) as specified in US Military Specification MIL-T-83133, JP-5 (flash point 60°C (140°F) min) as specified in U.S. Military Specification MIL-T-5642G, and Jet A and Jet A-1 (flash point 43.3°C (110°F) min) as specified in ASTM Specification D.1655/68.

[0024] At the copolymer concentrations indicated above, dissolution of the copolymer (as isolated, for example, by spray drying) in the liquid fuel may be effected by simple stirring or agitation, although it is desirable in many cases to heat the mixture at the same time, e.g. to a temperature of 80°C. When it is not practicable to isolate the copolymer in powder form, an alternative procedure for dissolving it in the liquid fuel is to add the aqueous latex slowly to the liquid fuel maintained at a temperature in the range 130-150°C, under which conditions the water from the latex is removed as an azeotrope.

[0025] The hydrocarbon fuels so modified are still liquids, in the sense that they have a viscosity of less than 1 poise (0.1 Pa. s), usually less than 0.1 poise (0.01 Pa . s).

[0026] There have previously been described, for example in Specifications GB-A-1 259 113, GB-A-1 285 197 and GB-A-1 332 593, liquid hydrocarbon fuels which, by virtue of their containing in solution other specified polymers or copolymers in defined proportions, possess improved resistance to particulate dissemination under shock conditions. More particularly, in Specification GB-A-1 337 288 there are described, as additives for liquid hydrocarbon fuels, addition copolymers of tert-butylstyrene with comonomers containing acidic groups, including acrylic acid and methacrylic acid, and with other monomers including acrylic esters, styrene and vinyltoluene. However, the particular combination of monomers, and their relative proportions, which are used according to the present invention are not disclosed therein.

[0027] Certain of the modified liquid hydrocarbon fuels of the present invention possess an advantage over the modified liquid fuels of the above-mentioned prior art in respect not only of their lower absolute viscosities but also of their low dependence of flow characteristics upon temperature. These advantages are particularly apparent in the case where the fuels contain copolymers according to the invention as hereinabove defined having the monomer composition (i) 81% to 85% of tert-butylstyrene, (ii) 9% to 11 % of methyl methacrylate and (iii) 6% to 8% of methacrylic acid. By way of illustration of this reduced temperature dependence of flow characteristics, there may be quoted the following comparative data for the efficiency of pumping, at two different temperatures, of a solution in aviation kerosene of a copolymer according to the invention and of a solution of the same concentration in the same kerosene of a copolymer according to the prior art. The pumping efficiency is calculated as the ratio of output hydraulic power to input electrical power when the liquids in question are delivered by an electrically driven two-stage impeller pump of the type fitted to many aircraft.

[0028] It will be seen that the pumping efficiency for kerosene modified according to the present invention is unimpaired under low temperature conditions, whereas there is a marked falling-off in efficiency for kerosene modified according to the prior art.

[0029] The invention is illustrated but not limited by the following Examples, in which parts are by weight.

Examples 1-18

General procedure

[0030] To a polymerisation vessel fitted with nitrogen inlet extending below the liquid level there was charged the following:

[0031] This charge was stirred until the surfactant had completely dissolved. There was then added 360 parts of a pre-mixed charge of monomer having a percentage composition as shown in the table below. The temperature of the reaction mixture was adjusted to 25°C and a nitrogen sparge was started at the rate of 200 ml of nitrogen per minute per Kg of total charge. After nitrogen had been passed for 5 minutes, there were added in the order stated the following initiator charges, prepared immediately prior to addition by dissolving each solid in the water:-

[0032] The nitrogen flow rate was then reduced to 50 ml per minute per Kg of total charge, and the temperature was maintained within the range 25-30°C for a total period of 6 hours counted from the addition of the initiators, cooling as necessary in order to control the reaction exotherm. Finally there was added 18 parts of a 0.1 % solution of n-octyl mercaptan and stirring and nitrogen flow were then discontinued.

[0033] There was thus obtained an aqueous copolymer emulsion of solids content approximately 20% by weight and average particle diameter 0.05-0.1 µm. The relative viscosity and the differential orifice flow rate of each copolymer obtained according to this general procedure, as a 0.3% solution in AVTUR 50, is shown in the table below. The copolymer solutions were obtained by adding the requisite proportion of the aqueous emulsion to AVTUR 50 at 130-150°C and removing the water as an azeotrope, then adjusting the concentration.

Examples 19-22

[0034] The general procedure described in Example 1 was repeated, but with variation in certain details as set out in the table below. The monomer composition in all cases was tert-butyl styrene/methyl methacrylate/methacrylic acid=83/10/7.

Examples 23-28

[0035] A number of the copolymers, the preparation of which is described in the preceding Examples, were also tested for their ability to confer resistance to misting and ignition under simulated aircraft crash conditions. A series of solutions of each polymer in AVTUR 50, having concentrations ranging from 0.0596 to 1% by weight, was prepared and these were subjected to test in an apparatus consisting of a small trolley guided along a track and fitted with a propulsion unit capable of accelerating the trolley to a speed of approximately 120 ft/sec (36.6 m/sec). The trolley is coupled to a braking system which is capable of stopping the trolley at a mean deceleration of 30 times the acceleration of gravity. A fuel tank is attached to the trolley and at the forward end of the fuel tank is an orifice which is closed with a weighted rubber bung. Approximately 45 mls of the fuel to be tested are placed in the tank and the trolley is winched back to a release point from which it is released and accelerated up to a speed of 120 ft/sec (36.6 m/sec). The acceleration takes place along about 10 feet (3.048 m) of the track and the trolley is then decelerated along about 10 feet (3.048 m) of the track by the braking system so that the weighted bung is ejected and the fuel is expelled through the tank orifice.

[0036] There is an ignition array of small gas flames spaced linearly at one foot (0.3048 m) intervals beneath the portion of the track over which deceleration takes place and beyond the track.

[0037] When unmodified AVTUR fuel was subjected to the test it produced a flare above the ignition array of 6-7 feet (1.83-2.13 m) in length and of large volume. On the other hand, when modified AVTUR fuel according to the invention was subjected to the same conditions, a concentration of copolymer of 0.196 or less was found to be effective in preventing any substantial ignition of the fuel. Details of the individual copolymers tested and of the results obtained are shown in the table below.

Example 29

[0038] The general procedure described in Examples 1-18 was repeated but with the following modifications in detail:-

(i) the amounts of acetone and distilled water in the initial charge were 144 parts and 1296 parts respectively, giving an acetone:water ratio of 1:9 instead of 1:4 as in the earlier case;

(ii) the sodium dithionite (0.3 part) was replaced by L; Ascorbic acid (0.27 part);

(iii) the temperature of polymerisation was 30-35°C instead of 25-30°C;

(iv) there was used as chain terminator 27 parts of a 0.1% solution of tert-dodecyl mercaptan in place of the 18 parts of a 0.1% solution of n-octyl mercaptan.

[0039] The monomer mixture polymerised under these conditions was that employed in Example 1 above. A 0.3% solution of the resulting copolymer in AVTUR 50 had the following characteristics:

Relative viscosity: 1.43

Differential orifice flow rate: 4.2 cm³/30 s.

Claims

1. A copolymer of (i) at least 75% by weight of tert-butylstyrene, (ii) from 1 % to 24% by weight of a second monomer selected from the acrylic and methacrylic esters of aliphatic monohydric alcohols containing from 1 to 4 carbon atoms, 2-ethoxyethyl methacrylate, acrylonitrile, vinyl acetate, styrene and vinyl toluene, and (iii) from 1% to 10% by weight of methacrylic acid, the aggregate of monomers (i), (ii) and (iii) being 100%, the copolymer having the following characteristics:

(A) solutions of the copolymer in AVTUR 50 aviation kerosene, at all concentrations in the range 0.05% to 1% by weight, are, notwithstanding that they may appear hazy or opalescent, nevertheless homogeneous in the sense that no separation from them of a swollen polymer phase occurs on standing at 20°C, AVTUR 50 being a liquid hydrocarbon fuel complying with U.K. Government Specification D. Eng. RD 2494 (NATO Code No. F-35);

(B) A 0.3% by weight solution of the copolymer in AVTUR 50 aviation kerosene has a relative viscosity in the range 1.3 to 2.1 and a differential orifice flow rate in the range 3 to 6 cm³ per 30 seconds, the said relative viscosity being the ratio of (a) the viscosity of the 0.3% by weight copolymer solution in AVTUR 50 when measured by the method of British Standard No. 188:1937 "The Determination of the Viscosity of Liquids in C.G.S. Units", Part 2, using a U-tube viscometer, Size A, at 25°C, to (b) the viscosity of AVTUR 50 when measured under those same conditions, and the said differential orifice flow rate being the difference between (c) the flow rate of a 0.3% by weight solution of the copolymer in AVTUR 50 through a passage of circular cross-section having a square-edged orifice, the passage having a length of 1.574 mm (0.062 inches) and a diameter of 0.635 mm (0.035 inches), and (d) the flow rate through the same said passage of a Newtonian liquid having the same viscosity as that of the copolymer solution referred to in (c) when the said viscosities are measured by the method of British Standard No. 188:1937, the flow rates being expressed as the volume of liquid in cm³ which passes through the orifice during the second period of 30 seconds of flow.

2. A copolymer as claimed in claim 1, wherein the second monomer is methyl methacrylate.

3. A copolymer as claimed in claim 1 having the monomer composition (i) from 75% to 90% by weight of tert-butyl styrene, (ii) from 7% to 15% by weight of methyl methacrylate and (iii) from 3% to 10% of methacrylic acid.

4. A copolymer as claimed in claim 3 having the monomer composition (i) from 81 % to 85% by weight of tert-butyl styrene, (ii) from 9% to 11 % by weight of methyl methacrylate and (iii) from 6% to 8% by weight of methacrylic acid.

5. A copolymer as claimed in claim 1 having a differential orifice flow rate in the range 4.0 to 5.5 cm³ per 30 seconds.

6. A process for the production of a copolymer which comprises the emulsion polymerisation in an aqueous diluent of a mixture of monomers consisting of (i) at least 75% by weight of tert-butyl- styrene, (ii) from 1 % to 24% by weight of a second monomer selected from the acrylic and methacrylic esters of aliphatic monohydric alcohols containing from 1 to 4 carbon atoms, 2-ethoxy-ethyl methacrylate, acrylonitrile, vinyl acetate, styrene and vinyltoluene, and (iii) from 1 % to 10% by weight of methacrylic acid, the aggregate of (i), (ii) and (iii) being 100%, the copolymer having the characteristics of solubility, relative viscosity and differential orifice flow rate in AVTUR 50 as defined in claim 1 and the polymerisation being characterised by the following features:-

(a) the aqueous diluent is a mixture of acetone and water in the ratio of from 1:9 to 1:2 by weight;

(b) the ratio of the weight of monomer mixture being polymerised to the weight of aqueous diluent is from 1:9 to 1:1.5;

(c) there is present in the reaction mixture an anionic surface-active agent at a concentration in the range 1% to 10% by weight of monomer mixture being polymerised;

(d) the reaction mixture is stirred at a temperature between 20°C and 45°C for a period of from 6 to 10 hours in the presence of a nitrogen atmosphere;

(e) there is added to the reaction mixture at some point during the first hour, in an amount of 0.05% to 0.3% based on the weight of monomer mixture, a redox initiator;

(f) there is added to the reaction mixture when polymerisation of the monomers is complete, in an amount of 0.001 to 0.1 % based on the weight of monomer mixture taken, a chain transfer agent.

7. A process as claimed in claim 6, wherein the aqueous diluent is a mixture of acetone and water in the ratio of 1:4 by weight.

8. A process as claimed in claim 6 or claim 7, wherein the ratio of the weight of monomer mixture to the weight of aqueous diluent is 1:4.

9. A process as claimed in any one of claims 6 to 8, wherein the anionic surface-active agent is sodium dioctyl sulphosuccinate used at a concentration of 2.5% of the weight of monomer mixture.

10. A process as claimed in any one of claims 6 to 9, wherein the reaction mixture is stirred at a temperature in the range 25-30°C.

11. A process as claimed in any one of claims 6 to 10, wherein the redox initiator is a mixture of ammonium persulphate and sodium dithionite, in the amounts of 0.05% and 0.075% respectively based on the weight of monomer mixture, and is added during the first 5 to 15 minutes of reaction time.

12. A process as claimed in any one of claims 6 to 11, wherein the chain transfer agent is n-octyl mercaptan, in an amount of 0.005% based on the weight of monomer mixture taken.

13. A copolymer of (i) at least 75% by weight of tert-butylstyrene, (ii) from 1% to 24% by weight of a second monomer selected from the acrylic and methacrylic esters of aliphatic monohydric alcohols containing from 1 to 4 carbon atoms, 2-ethoxyethyl methacrylate, acrylonitrile, vinyl acetate, styrene and vinyl toluene, and (iii) from 1 % to 10% by weight of methacrylic acid, the aggregate of monomers (i), (ii) and (iii) being 100%, characterised by being obtainable by the process of any one of claims 6 to 12.

14. A modified liquid hydrocarbon fuel of flashpoint of at least 32.8°C (90°F) suitable for use in gas turbine engined aircraft, and having a reduced tendency to particulate dissemination on being subjected to shock, the fuel containing dissolved therein from 0.05% to 1 % by weight of a copolymer of tert-butylstyrene as claimed in any one of claims 1 to 5 or claim 13.

15. A modified fuel as claimed in claim 14, containing from 0.2% to 0.5% by weight of the dissolved copolymer.

16. A modified fuel as claimed in claim 14 or claim 15, wherein the fuel is AVTUR 50 aviation kerosene as hereinbefore defined, aviation turbine fuel JP-8 (flashpoint 43.3°C (110°F) minimum) as specified in U.S. Military Specification MIL-T-83133, aviation turbine fuel JP-5 (flashpoint 60°C (140°F) minimum) as specified in U.S. Military Specification MIL-T-5624G, or Jet A or Jet A-1 (flashpoint 43.3°C (11 OOF) minimum) as specified in ASTM Specification D.1655/68.

Revendications

1. Copolymère (i) d'au moins 75% en poids de t-butylstyrène, (ii) de 1 à 24% en poids d'un second monomère choisi parmi les esters acryliques et méthacryliques d'alcools aliphatiques monohydroxylés comptant 1 à 4 atomes de carbone, le méthacrylate de 2-éthoxyéthyle, l'acrylonitrile, l'acétate de vinyle, le styrène et le vinyltoluène et (iii) de 1 à 10% en poids d'acide méthacrylique, la somme des monomères (i), (ii) et (iii) étant de 100%, le copolymère ayant les caractéristiques suivantes:

(A) les solutions du copolymère dans le kérosène pour aviation AVTUR 50, en toutes concentrations de l'intervalle de 0,05 à 1% en poids, sont, bien qu'elles puissent apparaître opalescentes ou troubles, néanmoins homogènes en ce sens qu'aucune séparation d'une phase de polymère gonflé ne s'y fait par repos à 20°C, l'AVTUR 50 étant un combustible hydrocarboné liquide satisfaisant à la spécification du Gouvernement du Royaume-Uni D. Eng. RD 2494 (NATO Code no. F-35);

(B) une solution à 0,3% en poids du copolymère dans le kérosène pour aviation AVTUR 50 a une viscosité relative de l'intervalle de 1,3 à 2,1 et un débit différentiel à l'orifice de l'intervalle de 3 à 6 cm³ par 30 secondes, cette viscosité relative étant le rapport (a) de la viscosité de la solution à 0,3% en poids du copolymère dans l'AVTUR 50, mesurée suivant le procédé de la norme B.S. no. 188:1937 "The Determination of the Viscosity of Liquids in C.G.S. Units", Partie 2, à l'aide d'un viscosimètre à tube en U de format A à 25°C, à (b) la viscosité de l'AVTUR 50, mesurée dans les mêmes conditions, et le débit différentiel à l'orifice étant la différence entre (c) le débit d'une solution à 0,3% en poids du copolymère dans l'AVTUR 50 par un passage à section transversale circulaire présentant un orifice à bord carré, le passage ayant une longueur de 1,574 mm (0,062 pouce) et un diamètre de 0,635 mm (0,025 pouce) et (d) le débit, par le même passage, d'un liquide newtonien ayant la même viscosité que celle de la solution de copolymère mentionnée en (c), les viscosités étant mesurées suivant le procédé de la norme B.S. no. 188:1937, les débits étant exprimés sous la forme du volume de liquide en cm³ passant par l'orifice au cours de la deuxième période de 30 secondes d'écoulement.

2. Copolymère suivant la revendication 1, dans lequel le second monomère est le méthacrylate de méthyle.

3. Copolymère suivant la revendication 1, ayant la composition en monomères (i) de 75 à 90% en poids de t-butylstyrène, (ii) de 7 à 15% en poids de méthacrylate de méthyle et (iii) de 3 à 10% d'acide méthacrylique.

4. Copolymère suivant la revendication 3, ayant la composition en monomères (i) de 81 à 85% en poids de t-butylstyrène, (ii) de 9 à 11% en poids de méthacrylate de méthyle et (iii) de 6 à 8% en poids d'acide méthacrylique.

5. Copolymère suivant la revendication 1, ayant un débit différentiel à l'orifice de 4,0 à 5,5 cm³ par 30 secondes.

6. Procédé de production d'un copolymère qui comprend la polymérisation en émulsion, dans un diluant aqueux, d'un mélange de monomères consistant (i) en au moins 75% en poids de t-butylstyrène, (ii) en 1 à 24% en poids d'un second monomère choisi parmi les esters acryliques et méthacryliques d'alcools aliphatiques monohydroxylés comptant 1 à 4 atomes de carbone, le méthacrylate de 2-éthoxyéthyle, l'acrylonitrile, l'acétate de vinyle, le styrène et le vinyltoluène et (iii) en 1 à 10% en poids d'acide méthacrylique, la somme de (i), (ii) et (iii) étant de 100%, le copolymère ayant les caractéristiques de solubilité, de viscosité relative et de débit différentiel à l'orifice dans l'AVTUR 50 telles que définies dans la revendication 1, et la polymérisation étant caractérisée par les particularités suivantes:

(a) le diluant aqueux est un mélange d'acétone et d'eau dans un rapport de 1:9 à 1:2 en poids;

(b) le rapport du poids du mélange des monomères en cours de polymérisation au poids du diluant aqueux est de 1:9 à 1:1,5;

(c) il y a en présence dans le mélange de réaction un agent surfactif anionique en une concentration de l'intervalle de 1 à 10% du poids du mélange des monomères en cours de polymérisation;

(d) le mélange de réaction est agité à une température entre 20°C et 45°C pendant une durée de 6 à 10 heures en présence d'une atmosphère d'azote;

(e) le mélange de réaction est additionné, à un certain moment au cours de la première heure, d'un initiateur redox en une quantité de 0,05% à 0,3% sur la base du poids du mélange des monomères;

(f) le mélange de réaction est additionné, lorsque la polymérisation des monomères est achevée, d'un agent de transfert de chaîne en une quantité de 0,001 à 0,1% sur la base du poids du mélange des monomères utilisés.

7. Procédé suivant la revendication 6, dans lequel le diluant aqueux est un mélange d'acétone et d'eau dans le rapport de 1:4 en poids.

8. Procédé suivant la revendication 6 ou 7, dans lequel le rapport du poids du mélange des monomères au poids du diluant aqueux est de 1:4.

9. Procédé suivant l'une quelconque des revendications 6 à 8, dans lequel l'agent surfactif anionique est le dioctylsulfosuccinate de sodium utilisé en une concentration de 2,5% du poids du mélange des monomères.

10. Procédé suivant l'une quelconque des revendications 6 à 9, dans lequel le mélange de réaction est agité à une température de l'intervalle de 25 à 30°C.

11. Procédé suivant l'une quelconque des revendications 6 à 10, dans lequel l'initiateur redox est un mélange de persulfate d'ammonium et de dithionite de sodium, en les quantités de 0,05% et 0,075% respectivement sur la base du poids du mélange des monomères et est ajouté pendant les 5 à 15 premières minutes de la durée de réaction.

12. Procédé suivant l'une quelconque des revendications 6 à 11, dans lequel l'agent de transfert de chaîne est le n-octylmercaptan en une quantité de 0,005% sur la base du poids des monomères utilisés.

13. Copolymère (i) d'au moins 75% en poids de t-butylstyrène, (ii) de 1 à 24% en poids d'un deuxième monomère choisi parmi les esters acryliques et méthacryliques d'alcools aliphatiques monohydroxylés comptant 1 à 4 atomes de carbone, le méthacrylate de 2-éthoxyéthyle, l'acrylonitrile, l'acétate de vinyle, le styrène et le vinyltoluène et (iii) de 1 à 10% en poids d'acide méthacrylique, la somme des monomères (i), (ii) et (iii) étant de 100%, caractérisé en ce qu'il peut être obtenu par le procédé suivant l'une quelconque des revendications 6 à 12.

14. Combustible hydrocarboné liquide modifié d'un point d'éclair d'au moins 32,8°C (90⁰F) convenant pour les appareils d'aviation à turbine à gaz et ayant une tendance réduite à la dissémination en particules lors de l'exposition à un choc, le combustible contenant en solution 0,05% à 1 % en poids d'un copolymère de t-butylstyrène suivant l'une quelconque des revendications 1 à 5 ou 13.

15. Combustible modifié suivant la revendication 14, contenant 0,2% à 0,5% en poids du copolymère dissous.

16. Combustible modifié suivant la revendication 14 ou 15, dans lequel le combustible est le kérosène pour aviation AVTUR 50 tel que défini ci-dessus, le combustible pour turbines d'aviation JP-8 (point d'éclair 43,3°C (1100F) au minimum) comme spécifié dans la spécification militaire des Etats-Unis d'Amérique MIL-T-83133, le combustible pour turbines d'aviation JP-5 (point d'éclair 60°C (140°F) au minimum) comme spécifié dans la spécification militaire des Etats-Unis d'Amérique MIL-T-5624G ou Jet A ou JET A-1 (point d'éclair 43,3°C (110°F) au minimum) comme spécifié dans la norme ASTM D.1655 68.

Ansprüche

1. Copolymer aus (i) mindestens 75 Gew.-% tert-Butylstyrol, (ii) 1 bis 24 Gew.-% eines zweiten Monomers, das ausgewählt ist aus Acryl- und Methacrylsäureestern von aliphatischen einwertigen Alkoholen mit 1 bis 4 Kohlenstoffatome, 2-Äthoxyäthylmethacrylat, Acrylonitril, Vinylacetat, Styrol-und Vinyltoluol, und (iii) 1 bis 10 Gew.-% Methacrylsäure, wobei die Monomere (i), (ii) und (iii) zusammengenommen 100% ausmachen und wobei das Copolymer die folgenden Charackteristiken aufweist:

(A) Lösungen des Copolymers in AVTUR 50-Flugzeugkerosin sind in allen Konzentrationen im Bereich von 0,05 bis 1 Gew.-96, obwohl sie trübe oder opaleszierend erscheinen können, homogen in dem Sinn, daß beim Stehen bei 20°C keine Abscheidung einer gequollenen Polymerphase aus ihnen auftritt, wobei AVTUR 50 ein flüssiger Kohlenwasserstofftreibstoff ist, der der U.K. Government Specification D.Eng. RD 2494 (NATO Code No. F-35) entspricht.

(B) Eine 0,3-gewichtsprozentige Lösung des Copolymers in AVTUR 50-Flugzeugkerosin besitzt eine relative Viskosität im Bereich von 1,3 bis 2,1 und einen Unterschied der Ausströmungsgeschwindigkeit aus einer Öffnung im Bereich von 3 bis 6 cm³/30 s, wobei die relative Viskosität das Verhältnis von (a) der Viskosität der 0,3-gewichtsprozentigen Copolymerlösung in AVTUR 50, gemessen durch die Methode von British Standard Nr. 188:1937 "The Determination of the Viscosity of Liquids in C.G.S. Units", Teil 2, unter Verwendung eines U-Rohr-Viskosimeters, Größe A, bei 25°C, zu (b) der Viskosität von AVTUR 50, gemessen unter den gleichen Bedingungen, ist und wobei der Unterschied der Ausströmungsgeschwindigkeit aus einer Öffnung der Unterschied ist zwischen (c) der Strömungsgeschwindigkeit einer 0,3-gewichtsprozentigen Lösung des Copolymers in AVTUR 50 durch einen Kanal mit kreisförmigem Querschnitt und mit einer rechtwinklige Ränder aufweisenden Öffnung, wobei der Kanal eine Länge von 1,574 mm (0,062 Inch) und einen Durchmesser von 0,635 mm (0,035 Inch) aufweist, und (d) der Strömungsgeschwindigkeit einer Newtonschen Flüssigkeit mit der gleichen Viskosität als diejenige der in (c) genannten Copolymerlösung durch den gleichen Kanal, wobei die Viskositäten gemessen sind durch die Methode von British Standard Nr. 188:1937, und die Strömungsgeschwindigkeiten ausgedrückt sind als das Volumen Flüssigkeit in cm³, das durch die Öffnung während der zweiten von 30 s dauernden Strömungsperioden hindurchgeht.

2. Copolymer nach Anspruch 1, bei welchem das zweite Monomer aus Methylmethacrylat besteht.

3. Copolymer nach Anspruch 1 mit der Monomerzusammensetzung (i) 75 bis 90 Gew.-% tert-Butylstyrol, (ii) 7 bis 15 Gew.-% Methylmethacrylat und (iii) 3 bis 10 Gew.-% Methacrylsäure.

4. Copolymer nach Anspruch 3 mit der Monomerzusammensetzung (i) 81 bis 85 Gew.-% tert-Butylstyrol, (ii) 9 bis 11 Gew.-% Methylmethacrylat und (iii) 6 bis 8 Gew.-% Methacrylsäure.

5. Copolymer nach Anspruch 1 mit einem Unterschied der Ausströmungsgeschwindigkeit aus einer Öffnung im Bereich von 4,0 bis 5,5 cm³/30 s.

6. Verfahren zur Herstellung eines Copolymers durch Emulsionspolymerisation eines Monomerengemischs in einem wäßrigen Verdünnungsmittel, wobei das Monomerengemisch besteht aus (i) mindestens 75 Gew.-% tert-Butylstyrol, (ii) 1 bis 24 Gew.-% eines zweiten Monomers, das ausgewählt ist aus Acryl- und Methacrylsäureestern von aliphatischen einwertigen Alkoholen mit 1 bis 4 Kohlenstoffatomen, 2-Äthoxyäthylmethacrylat, Acrylonitril, Vinylacetat, Styrol- und Vinyltoluol, und (iii) 1 bis 10 Gew.-% Methacrylsäure und die Monomere (i), (ii) und (iii) zusammengenommen 100% ausmachen, wobei das Copolymer die in Anspruch 1 definierten Charakteristiken der Löslichkeit, der relativen Viskosität und des Unterschieds der Ausströmungsgeschwindigkeit aus einer Öffnung in AVTUR 50 besitzt, wie sie in Anspruch 1 definiert sind, und wobei die Polymerisation durch die folgenden Merkmale gekennzeichnet ist:

(a) Das wäßrige Verdünnungsmittel ist ein Gemisch aus Aceton und Wasser im Gewichtsverhältnis von 1:9 bis 1:2.

(b) Das Verhältnis des Gewichts des zu polymerisierenden Monomergemischs zum Gewicht des wäßrigen Verdünnungsmittels beträgt 1:9 bis 1:1,5.

(c) Im Reaktionsgemisch ist ein anionisches oberflächenaktives Mittel in einer Konzentration im Bereich von 1 bis 10 Gew.-%, bezogen auf das zu polymerisierende Monomerengemisch, anwesend.

(d) Das Reaktionsgemisch wird bei einer Temperatur zwischen 20 und 45°C während einer Zeit von 6 bis 10 h in Gegenwart einer Stickstoffatmosphäre gerührt.

(e) Dem Reaktionsgemisch wird zu irgendeinem Zeitpunkt während der ersten Stunde ein Redoxinitiator in einer Menge von 0,05 bis 0,3%, bezogen auf das Gewicht des Monomerengemischs, zugegeben.

(f) Dem Reaktionsgemisch wird nach beendeter Polymerisation der Monomere ein Kettenübertragungsmittel in einer Menge von 0,001 bis 0,1%, bezogen auf das Gewicht des verwendeten Monomerengemischs, zugegeben.

7. Verfahren nach Anspruch 6, bei welchem das wäßrige Verdünnungsmittel ein Gemisch aus Aceton und Wasser im Gewichtsverhältnis von 1:4 ist.

8. Verfahren nach Anspruch 6 oder 7, bei welchem das Verhältnis des Gewichts des Monomerengemischs zum Gewicht des wäßrigen Verdünnungsmittels 1:4 beträgt.

9. Verfahren anch einem der Ansprüche 6 bis 8, bei welchem das anionische oberflächenaktive Mittel Natriumdioctylsulfosuccinat ist, das in einer Konzentration von 2,5%, bezogen auf das Gewicht des Monomerengemischs, verwendet wird.

10. Verfahren nach einem der Ansprüche 6 bis 9, bei welchem das Reaktionsgemisch bei einer Temperatur im Bereich von 25 bis 30°C gerührt wird.

11. Verfahren nach einem der Ansprüche 6 bis 10, bei welchem der Redoxinitiator ein Gemisch aus Ammaniumpersulphat und Natriumdithionit in Mengen von 0,05 % bzw. 0,075%, bezogen auf das Gewicht des Monomerengemischs, ist und während der ersten 5 bis 15 min der Reaktionszeit zugegeben wird.

12. Verfahren nach einem der Ansprüche 6 bis 11, bei welchem des Kettenübertragungsmittel n-Octylmercaptan in einer Menge von 0,005%, bezogen auf das Gewicht des verwendeten Monomerengemischs, ist.

13. Copolymer aus (i) mindestens 75 Gew.-% tert-Butylstyrol, (ii) 1 bis 24 Gew.-% eines zweiten Monomers, das ausgewählt ist aus Acryl- und Methacrylsäureestern von aliphatischen einwertigen Alkoholen mit 1 bis 4 Kohlenstoffatomen, 2-Äthoxyäthylmethacrylat, Acrylonitril, Vinylacetat, Styrol und Vinyltoluol, und (iii) 1 bis 10 Gew.-% Methacrylsäure, wobei die Monomere (i), (ii) und (iii) zusammengenommen 100% ausmachen, dadurch gekennzeichnet, daß es durch das Verfahren nach einem der Ansprüche 6 bis 12 erhältlich ist.

14. Modifizierter flüssiger Kohlenwasserstofftreibstoff mit einem Flammpunkt von mindestens 32,8°C (90°F), der sich für die Verwendung in einem Flugzeug mit Gasturbinenantrieb eignet und der beim Auftreten eines Schocks eine verringerte Neigung zur Ausschleuderung von Teilchen aufweist, wobei im Treibstoff 0,05 bis 1 Gew.-% eines tert-Butylstyrolcopolymers nach einem der Ansprüche 1 bis 5 oder 13 aufgelöst ist.

15. Modifizierter Treibstoff nach Anspruch 14, der 0,2 bis 0,5 Gew.-% des aufgelösten Copolymers enthält.

16. Modifizierter Treibstoff nach Anspruch 14 oder 15, bei dem der Treibstoff aus AVTUR 50-Flugzeugkerosin gemäß obiger Definition, Flugzeugturbinentreibstoff JP-8 (Flammpunkt 43,3°C (110°F) mindestens), wie er in U.S. Military Specification MIL-T-83133 spezifiziert ist, Flugzeugturbinentreibstoff JP-5 (Flammpunkt 60°C (140°F) mindestens), wie er in U.S. Military Specification MIL-T-5624G spezifiziert ist, oder Jet A oder Jet A-1 (Flammpunkt 43,3°C (110°F) mindestens), wie er in ASTM Specification D. 1655/68 spezifiziert ist, besteht.